1. Technical Field
The present invention relates to a microelectromechanical (MEMS) sensor having multiple full-scale and sensitivity values. In particular, in what follows reference will be made, without this, however, implying any loss in generality, to an accelerometer sensor.
2. Description of the Related Art
As is known, microelectromechanical sensors made in semiconductor technology are today used in a wide range of technological sectors, thanks to their small dimensions, versatility, and low costs. In particular, accelerometer sensors today find a wide range of uses in the “automotive” field, for example in airbag systems, in stability-control systems (ESP®—Electronic Stability Program) and in systems for brake-assistance (ABS—Antilock Braking System).
In a known way, accelerometers for airbag-control systems have a measuring scale with high full-scale values (for example, 50 g, where g is gravitational acceleration), whilst lower full-scale values (for example, between 2 g and 6 g) are in accelerometers for stability-control and brake-assistance systems. The full-scale value of these sensors, in addition to indicating a maximum detectable value, is in general associated to the sensitivity of the sensors, since the dynamic range of a corresponding output signal is fixed (for example, comprised between 0 and 5 V, in the case of a voltage output signal). In other words, an increase in the full-scale value is to be considered as equivalent to a decrease in the sensitivity of the sensor (and vice versa), and a sensor having a low full-scale value has a high sensitivity (and vice versa).
Currently, in MEMS accelerometer sensors the implementation of different full-scale (or sensitivity) values is entrusted to a corresponding reading electronics, which achieves this by varying a gain factor of an amplification stage in the signal-processing chain.
In detail, and as is shown schematically in FIG. 1, a conventional MEMS accelerometer 1 generally comprises a sensing structure 2 made with micromachining techniques of a semiconductor material and having a rotor (or mobile element) and a first stator and a second stator (or fixed elements). The first and second stators are capacitively coupled to the rotor so as to form a first sensing capacitor C1 and a second sensing capacitor C2 (in particular, the first and second stators face the rotor, forming plane parallel plate capacitors). The sensing structure 2 has at its output a first and second stator contacts S1, S2 and a rotor contact R, which constitute the terminals of the first and second sensing capacitors C1, C2. The rotor is free to be displaced (in particular by linear motion) with respect to the first and second stators as a function of the acceleration, consequently varying the value of capacitance of the first and second sensing capacitors C1, C2. From the differential capacitive unbalancing of the sensing capacitors, a reading electronics 4 coupled to the sensing structure 2 is able to determine the value of the acceleration acting on the MEMS accelerometer 1. In greater detail, the reading electronics 4 comprises: a processing stage 6 adapted to convert the capacitive unbalancing signal into an electrical signal and to filter and process this signal in a suitable way, and a gain stage 7, connected to the output of the processing stage 6, having a variable gain that can be selected via a full-scale selection command FS, that is provided at an input of the reading electronics 4. According to the requirements, by acting on the full-scale selection command FS, it is possible to vary the full-scale value of the MEMS accelerometer 1 (for example, between 2 g and 6 g, or else between 35 g and 50 g), and consequently to vary its sensitivity, so as to adapt it to different applications.
The described solution, albeit advantageous in so far as it enables easy variation of the full-scale value of the accelerometer sensor, has certain drawbacks, amongst which the difficulty of implementing a wide range of full-scale values by acting on the gain stage 7, and the non-linearity of the response of the MEMS accelerometer 1 for high full-scale values. The latter aspect is due to the fact that, as is known, the capacitance of a plane parallel plate capacitor is linear for small displacements (as compared to the distance, or gap, between its plates), whilst it is non linear for large displacements (comparable to the aforesaid distance between the plates).